The present invention relates generally to semiconductor diodes and, more particularly, to tunnel diodes fabricated of gallium arsenide.
A tunnel diode is a diode having a P-N junction formed between a heavily doped P region and a heavily doped N region of a semiconductor, one of the regions being very thin.
In accordance with well known principles, majority charge carriers can travel across such a junction despite an opposing potential barrier created by a voltage applied to the junction, a process known as "tunneling." Tunneling takes place only over a limited range of applied voltages, however, and as the forward voltage is increased, the barrier height decreases beyond a certain magnitude and current flow due to tunneling drops off rapidly. Such as inverse relationship between applied voltage and current flow is sometimes called a "negative resistance" characteristic. A negative resistance characteristic is desirable because a device having such a characteristic can be made to oscillate or to amplify. Tunnel diodes are especially useful because they combine a negative resistance characteristic with an ability to function well at microwave frequencies, and hence tunnel diodes find a variety of applications in microwave circuits as oscillators, amplifiers, and the like.
Tunnel diodes fabricated by means of planar technology (that is, formed in one side of a wafer of semiconductor material) have certain advantages over those made by means of mesa, ball-alloy, or electronic pulse techniques. Among these advantages are a more rugged and mechanically stable construction, and more economical manufacture because planar technology leads to larger volume, and a higher production yield. Also, planar tunnel diodes are more reliable and have more predictable electrical characteristics. Prior to this invention, gallium arsenide has not lent itself to planar fabrication techniques, and the only practical planar tunnel diodes have been made of germanium. Unfortunately, germanium planar tunnel diodes are not capable of operation at sufficiently high power or at sufficiently high temperatures to be usable for radio-frequency (rf) detector applications in flight. Tunnel diodes fabricated from gallium arsenide can function under these conditions, but some other construction technique not offering the advantages of planar construction has been used to make such diodes.
It will be apparent from the foregoing that there is a need for a planar tunnel diode that can be used in flight for rf detector applications or in other applications requiring gallium arsenide tunnel diodes. The present invention satisfies this need.